Protein kinases play key functions in cell signal transduction by phosphorylation of tyrosine, serine or threonine residues of proteins. They have become very attractive targets for therapeutic interventions in many disease states such as cancer, inflammation, arthritis and diabetes. Receptor protein tyrosine kinases have become compelling targets for cancer chemotherapy.
Tropomyosin-receptor kinase (Trk) receptors are a family of receptor tyrosine kinases that regulates synaptic strength and plasticity in the mammalian nervous system. Trk receptor signaling activates several small G proteins, including Ras, Rap-1, and the Cdc-42-Rac-Rho family, as well as pathways regulated by MAP kinase, PI 3-kinase and phospholipase-C-γ (PLC-γ). Huang, et al. Ann. Rev. Biochem. 72:609-642 (2003). Trk A and Trk C have been implicated in pancreatic cancer and colon cancer respectively; while Trk B is believed to be an attractive target for treatment of neuroblastoma, pancreatic cancer and colon cancer. See Sakamoto Y. et al. Oncol Rep. 2001, 8(3):477-84; Ma J. et al. J. Gastroenterol Hepatol. 2008, 23(12):1852-9; Dang C. et al. J. Gastroenterol Hepatol. 2006, 21(5):850-8; Okada Y. et al. Clin. Exp. Metastasis 2004, 21(4):285-92; Liu D. et al. Oncol. Rep. 2007, 18(3):673-7; Miknyoczki S. J. et al. Int. J. Cancer 1999, 81(3):417-27; Sasahira T. et al. Hum. Pathol. 2013, 44(6):1098-106; Asgharzadeh et al. J. Natl. Cancer Inst. 2006, 98(17):1193-203; Nakagawara A. el al. Mol. Cell. Biol. 1994, 14(1):759-67; Brodeur G. M. et al. Clin. Cancer Res. 2009, 15(10):3244-50; Ho R. et al. Cancer Res. 2002, 62(22):6462-6; Matsumoto K. et al. Cancer Res. 1995, 55(8):1798-806; Sclabas G. M. et al. Clin. Cancer Res. 2005, 11(2 Pt 1):440-9; Li Z. et al. Cancer Res. 2009, 69(19):7851-9; Sasahira T. et al. Hum. Pathol. 2013, 44(6):1098-106; Akil H. et al. PLoS One. 2011, 6(9); and Yu Y. et al. APMIS. 2010, 118(3):188-95.
The insulin-like growth factor-I receptor (IGF-IR) is a tyrosine kinase membrane receptor having a structure very similar to that of the insulin receptor (IR). The structure of IGF-IR consists of two extracellular α-chains that form the ligand-binding domain and two β-chains that make up the transmembrane and intracellular domains. IGF-IR is the primary receptor for insulin-like growth factor IGF-I, although IGF-II and insulin can also bind with less affinity. Upon ligand binding, IGF-IR is activated, resulting in autophosphorylation of tyrosines on the intracellular β-subunit. IGF-IR then phosphorylates intracellular proteins such as the insulin receptor substrates 1 to 4 (IRS1-IRS4) and Shc. These substrates, in turn, initiate phosphorylation cascades that activate the phosphatidylinositol 3-kinase (PI-3K)/protein kinase B (Akt) or mitogen-activated protein kinase (MAPK) pathways (Samani et al. Endocr. Rev. 28:20-47 (2007)).
Through activation of these signaling cascades, IGF-IR has been implicated in cancer. The exact role of IGF-IR in cancer, however, remains uncertain and appears to vary according to tumor or cell type. For example, some tumors may depend on IGF-IR signaling for survival, whereas others rely on IGF-IR for proliferation. Yet other tumors may employ IGF-IR overexpression as a mechanism of resistance against cytotoxic agents such as anti-cancer drugs (Rodon et al. Mol. Cancer Ther. 7:2575-2588 (2008)). Accordingly, inhibition of IGF-IR is an attractive drug strategy for cancer treatment.
Although IGF-IR was first cloned in the 1980s, drug development to target IGF-IR has been slow to develop. Currently, there are close to 30 drug candidates that target IGF-IR in various clinical phases including both monoclonal antibodies and small molecule tyrosine kinase inhibitors, but no molecule has yet received FDA approval for cancer treatment (Rodon et al. Mol. Cancer Ther. 7:2575-2588 (2008); Gualberto et al. Oncogene 28:3009-3021 (2009)). There remains a clear need to target IGF-IR through the development of potent inhibitors of this receptor.
Other protein kinases including those detailed herein are also important targets for treatment of conditions or disorders associated with protein kinases, such as cancer. A number of approved cancer therapeutics may function by targeting protein kinases. However, cancer remains a prevalent disease and there remains a need for new cancer therapeutics.